The present invention relates to the radio communications art. Specifically, a sigma delta modulator is described for processing the entire bandwidth of the cellular telephone GSM spectrum.
High speed analog to digital converters (ADC) are utilized to process higher IF frequency signals. In the portable telephone art, sigma delta modulators are used in analog to digital converters. The sigma delta ADCs receive an analog input signal which is subtracted from a feedback signal to produce an error signal. The resulting error signal is filtered and digitized using a digital quantizer. The resulting quantized signal is reconverted to an analog signal, and used as the feedback signal. The basic sigma delta analog to digital converter provides higher resolution because of higher clocking speeds which over sample the input signal. The basic sigma delta ADC also provides a signal having a high signal to noise ratio through the use of a low pass filter (or band pass filter) which shapes the quantization noise frequency spectrum to fall outside the frequency band of the quantized signal.
In radio telephone communications applications, it is desirable to process signals over the entire GSM intermediate frequency band. The IF frequency signal produced from frequency down conversion may include images produced from the down conversion process which are desirably removed to avoid any false signal detection.
The prior art employs various filtering circuits in the sigma delta modulation system. On chip integrated filters, however, produce noise and distortion at high intermediate frequency signals. The filters used in the delta sigma modulators include switched capacitors (SC), inductor/capacitors (LC), transconductance/capacitor (gm-c), active resistor/capacitors (RC) and MOS capacitor-type filters. FIG. 1 illustrates one type of off chip LC loop filter.
These filters which are necessary to remove images, and maintain a low noise/distortion of the intermediate frequency, may require a costly manual tuning process. Additionally, it is difficult to design gm-C filters that are highly linear, and which introduce minimal noise to the signal processing of the intermediate frequency signals. These problems are compounded by the higher frequencies i.e., above hundreds of megahertz frequency range utilized in the radio telecommunications art. The amplitude and phase nonlinearity of these filters can produce intermodulation products from signals within the intermediate frequency band. Accordingly, it is desirable to provide a filter for the sigma delta modulator which produces a low noise level, and has a low distortion output at higher intermediate frequency signal frequencies.
An analog-to-digital converter is described for processing intermediate frequency signals above the hundreds of megahertz range. The analog-to-digital converter is implemented as a sigma delta modulator which produces from an analog input signal a digitized output signal. The apparatus in accordance with the invention provides a signal delta modulator which receives on an input of a summing amplifier the analog intermediate frequency signal. The summing amplifier also receives a feedback signal derived from the digitized intermediate frequency signal. A surface acoustic wave filter (SAW) filters the signal from the summing amplifier, and eliminates image signals which were produced during the down conversion process. The SAW filter has a sufficient bandwidth to accommodate the entire GSM intermediate frequency spectrum and produced an image free, low noise output signal.
The SAW filter output signal is amplified and applied to a quantizer where it is digitized. A feedback loop is formed from the output of the quantizer circuit, and a second input of the summing amplifier. The feedback signal is provided by a digital to analog converter which converts the digitized intermediate frequency signal to an analog signal, and provides the analog signal as a feedback signal for the summing amplifier.
The SAW filter does not include any of the disadvantages of the traditional filtering techniques used in sigma delta modulators. Pass bandwidth, stop band attenuation, and bandwidth ripple are easily met using the SAW filter. Additionally, the SAW filter does not introduce any significant noise, or distortion products at the higher frequency intermediate frequency signals.